Electric wave-filter



March 24, 1936. A, JAUMANN 2,034,905

ELECTRIC WAVE FILTER Filed March 21, 1932 INVENTOR ANDREAS JAUMANN ATTORNEY Patented Mar. 24, 1936 UNITED STATES PATENT OFFICE ELECTRIC WAVE -FILTER Application March 21, 1932, Serial No. 600,110 In Germany March 1'7, 1931 9 Claims.

For a great many purposes in radio engineering, for instance, in broadcast reception, current smoothing means to eliminate ripples, reception in multiplex traffic along power lines by the aid of portable or transportable equipment and the like, it is desirable that the wave-filters (bandpass filters) used in connection therewith should be so adjustable that, similarly as in broadcast receiver sets, tuning to a desired frequency band should be accomplishable in a simple manner without the marginal or boundary frequencies being distorted as a result of the sloped droop of the resonance curve. In broadcast reception the demand for such a wave-filter grows with increase in the number of transmitters inasmuch as the frequency band available for any given program shrinks more and more as broadcasting stations increase in number. However, it is not only the preclusion of extraneous stations, but also the elimination of atmosphere disturbance, that makes reception appear desirable inside a sharply bounded frequency band. Now, filters have been suggested in the prior art whereby such reception is feasible. These filters consist of a plurality of oscillatory circuits of similar tuning which are in loose coupling relationship. According to one suggestion the condenser settings (controls) of the various circuits are to be mechanically intercoupled in such a fashion that the tuning of these band filters is accomplishable by the agency of a single knob or dial. However, filters of the said sort involve the drawback that their impedance is a function of the tuning. Another fact is that only the relative channel width remains constant upon detuning, whereas the absolute channel width must be set to the desired value by varying the coupling.

The invention is concerned with a wave-filter or band-pass filter which, for a given and fixed band width, makes it possible to vary the range of transmission throughout a wide frequency range without a change in the impedance at the input or the output end being caused. The wavefilter according to the invention consists of two branches or units being at one end united directly and on the other end by Way of an inductance coil or reactor. One pair of terminals (input) of this band-pass filter is constituted by the common point of the branches and the midpoint of the coil, whereas the second pair of terminals (output) is tapped on the coil upon both sides of the middle. Instead of the inductance coil (autotransformer) it is also feasible to employ the winding of a transformer, the respective other winding of the transformer serving as the output. Filters of this kind have been previously disclosed in U. S. Patent Number 1,882,631, though without mechanical coupling of the tuning means.

The invention will best be understood by referring to the accompanying drawing in which,

Figure 1 is a diagram of a Wave filter,

Figure 2 is a side view of two variable condensers having rotors mounted on a single shaft,

Figure 3 is a detail of two rotary plates of a 10 variable condenser, one plate being displaced slightly from the other in angular relationship,

Figure 4 is a wiring diagram of an arrangement employing an auxiliary condenser.

As shown in Fig. 1 the wave-filter consists of two impedances Z! and Z2, each one of which comprises at least a series arrangement of a coil and a condenser. Zl or Z2, or both, however, could consist also of a plurality of paralleled branches or units of this kind. In each resonance branch one element (coil or condenser) is disposed so as to be adjustable, this being insured in such a way that the setting of an element in a resonant branch is coupled with the adjustment of the element in the respective other branch or unit. If branches Zl and Z2 consist of the paralleling of several series arrangements comprising coil and condenser, then a variable impedance must be provided for each one of such paralleled resonance circuits or branches.

Fig. 1 illustrates the coupled adjustment for the two condensers in Z! and Z2. Inasmuch as the condensers of the series arrangements have a group of plates at common potential they may be readily mounted upon a common spindle as shown in Fig. 2. The rotor plates of the condensers in this scheme should be preferably shaped as shown for instance, in Fig. 3 so that the capacity is inversely proportional to the square of the angle of rotation. The frequency of an associated oscillation circuit, when condensers of the said kind known in the prior art are used, will be proportional to the angle of rotation. If Zl and Z2, Fig. 1, are formed by the series arrangement of a coil and a condenser the proportions of the circuit elements can be readily indicated as illustrated by the following example. In the same, for the sake of simplicity, the assumption is made that the coils of the two branches Zl and Z2 are alike and that the relative channel width is small or less than 0.1. For a given mean surge impedance of the filter (within the range of transmission) Z0 which must be adapted to the impedance of the associated line or receiver or antenna, for an absolute channel width Ao=wlw2 and a relative channel width if ml and (02 stand for resonant cyclic frequencies of the branches.

. 1 1 1 1 ho. If the capacity of the second circuit consists of a rotary condenser having plates of any desired shape at all (C2=f( )=angle of rotation), then in the light of the above condition the form of plates of the first condenser is given 1 Z A9 a 0 1/ o 2) 2 A Since in the case of small relative channel width (and this is the case most important in practice) the condenser values C1 and C2 differ but little from each other it will be convenient to choose two like condensers C2; while disparity in capacity is insured by means of a small supplementary condenser in. fixed coupling relationship with one of the main condensers, said supplemental condenser having this value c=c c,= 2,?cfl

The condition of constant channel width can be satisfied with still greater simplicity if condensers are employed whose capacity grows with the reciprocal square of the angle of rotation, thus In the light of what precedes there is obtained in this instance:

1 A m (Llg- 0(1) 01;"(11

Hence, in both branches like condensers of the prescribed form of plates may be used which while displaced a constant small angle in reference to each other, are in fixed coupling relationship. (Fig.3.)

In this embodiment moreover the absolute channel width of the filter can be varied throughout the entire coverable frequency range in a simple manner, in that the angle of adjustment of the coupling is altered, though this means also a positive change in the surge impedance of the filter-chain throughout the entire frequency band. 7

If several resonance branches in parallel connection are contained in the impedances ZI and Z2, then the condensers comprised in the resonant branches are subject to'the identical conditions. In this instance, similarly as indicated in Fig. 4 for one branch or mesh (Zl) it is possible by providing a small supplemental condenser which preferably consists of an adjustable auxiliary plate, to create conditions so that while in the filter the charmel width and the surge impedance are preserved in rough approximation, the slope of the damping increment at the boundaries of the channel may be adjusted in order that satisfactory conditions for the elimination of outer disturbances may still be made variable at will. The small auxiliary capacity Cx shown in Fig. 4, as will be noted, offers a chance to shift two resonant frequencies inside a the Wave-filter and which are located inside the range of transmission, more or less close to the channel limits. The idea. of influencing the slope of the growth in damping in this way in filters of the kind as here concerned has been suggested previously in U. S. Patent 1,882,631 and U. S. patent application Serial Number 566,750. The branch 22 must be similarly constructed and the auxiliary condenser must be mechanically intercoupled with the auxiliary condenser of branch 2| in such a fashion that the internal resonance frequencies are symmetrically shiftable away from the center of the channel towards the marginal frequencies.

In line with what. has been suggested in. another application for Letters Patent Serial Number 566,750 another series branch inside 25 or 22 may serve the purpose of making the surge impedance not only constant, but also evened inside a given range of transmission upon a shift of the range of transmission. The condensers of the branches or units added in that case must then likewise fulfill the mechanical conditions; of coupling of the other condensers.

The tuning of the resonance branches added for the. purpose of evening the surge impedance should agree with the fundamental frequencies of the range of. transmission, i. e., with the tuning. of the resonant branches governing the same. The resonant circuits of like tuning, however, arecontained in different, branches zl or 22.

Instead of choosing for the condensers a suitable form of. plate and, instead of coupling them by a common spindle, it is also feasible to use like condensers of any desired plateform' in the branch pairs corresponding to each other; though in that case attention must be given so that a mechanical gearing upon turning the various condensers will satisfy the laws of capacity variation previously laid down.

The connection of the branches 2! or 22 (see Fig. 1) or of the output must not be efiected at the ends of the transformer; it is furthermore not necessary that the various: resonance branches in el or 22 should be tapped at thesame point of the transformer. By choosing the tap,- ping point this affords a further meansto vary the surgeimpedance. The networks in resistance reciprocity to the filters described. also allow of the construction of adjustable filter-chai'nswith constant absolute band width and constant surge impedance. However, for these forms it is preferable to choose fixed capacities and variable inductances or variometers.

I claim:

1. A band-pass filter comprising two variable reactive branches which at one end are electrically associated and at the other end united with a transformer winding whose output is represented by a pair of terminals of the filter, whereas the other pair of terminals is constituted by the midpoint tap of the transformer winding and the common point of two branches, said two branches comprising adjustable rotatable condensers in the respective branches, the desired tuning being effected by means of an adjustable auxiliary condenser plate connected in parallel relationship with one of the adjustable rotatable condensers of said branches.

2. A band-pass filter comprising two variable reactive branches which at one end are electrically associated and at the other end united with a transformer winding whose output is represented by a pair of terminals of the filter, whereas the other pair of terminals is constituted by the midpoint tap of the transformer winding and the common point of two branches, said two branches comprising adjustable rotating condensers whose capacity value increases inversely proportional to the square of the angle of rotation.

3. A band-pass filter comprising two variable reactive branches which at one end are electrically associated and at the other end united with a transformer winding whose output is represented by a pair of terminals of the filter, whereas the other pair of terminals is constituted by the midpoint tap of the transformer winding and the common point of twobranches, said two branches comprising adjustable rotating condensers wherein the group of plates of each condenser is mounted upon a common spindle while the plates of the different condensers are displaced with reference to each other, a certain constant angle of rotation during the joint setting of the condensers.

4. A band-pass filter comprising two variable reactive branches which at one end are electrically associated and at the other end united with a transformer winding whose output is represented by a pair of terminals of the filter, whereas the other pair of terminals is constituted by the midpoint tap of the transformer winding and the common point of two branches, said two branches comprising a coil connected in series with adjustable rotatable condensers wherein each group of rotatable plates of said condensers are mechanically inter-coupled for adjustment to obtain resonance in the two branches, and at least one group of condenser plates having an auxiliary condenser connected in parallel relationship therewith.

5. A band pass filter having adjustable means whereby a shift in range of constant absolute band transmission width the value of surge impedance within the entire coverable frequency range is preserved, said adjustable means comprising a pair of adjustable reactances connected in parallel at one end and to one input terminal of the filter, the other end of said reactance connected in series with the primary of a transformer, the other input terminal of said filter being connected to a tap on said primary, the secondary of said transformer being connected to the output terminals of said filter.

ary of said transformer being connected to the output terminals of said filter, at least one of the adjustable reactances being formed by a condenser and a coil connected in series.

7. A band pass filter having adjustable means whereby a shift in range of constant absolute band transmission width the value of surge impedance within the entire coverable frequency range is preserved, said adjustable means comprising a pair of adjustable reactances connected in parallel at one end and to one input terminal of the filter, the other end of said reactance connected in series with the primary of a transformer, the other input terminal of said filter being connected to a tap on said primary, the secondary of said transformer being connected to the output terminals of said filter, and at least one of the adjustable reactances being formed by a variable condenser and a coil, said variable condenser having rotatable plates, the rotatable plates of each reactance being mechanically coupled.

8. A band pass filter having adjustable means whereby'a shift in range of constant absolute band transmission width the value of surge impedance within the entire coverable frequency range is preserved, said adjustable means comprising a pair of adjustable reactances connected in parallel at one end and to one input terminal of the filter, the other end of said reactance connected in series with the primary of a transformer, the other input terminal of said filter being connected to a tap on said primary, the secondary of said transformer being connected to the output terminals of said filter, and at least one of the adjustable reactances being formed by a condenser and a coil connected in series and an auxiliary condenser connected in parallel with the condenser which is connected in series with the coil.

9. A band pass filter having adjustable means whereby a shift in range of constant absolute band transmission width the value of surge impedance within the entire coverable frequency range is preserved, said adjustable means comprising a pair of adjustable reactances connected in parallel at one end and to one input terminal of the filter, the other end of said reactance connected in series with the primary of a transformer, the other input terminal of said filter being connected to a tap on said primary, the second ary of said transformer being connected to the output terminals of said filter, at least one of the adjustable reactances being formed by a condenser and a coil connected in series and an auxiliary condenser having asmall capacity with respect to the capacity of the condenser in said reactance and connected in parallel therewith.

ANDREAS J AUMANN 

